This invention relates to a method and apparatus for reducing air polluting agents such as nitrogen oxides, carbon monoxide, carbonaceous particulates and unburned hydrocarbons in the exhaust gases of carbonaceous and hydrocarbon fuel burners.
The conventional method of burning gaseous, liquid and finely divided solid fuels is to inject the fuel and air in excess of the stoichiometric amount into a combustion chamber under conditions which promote intimate mixing of the air and fuel and supposedly more complete combustion of the fuel. Generally, the objective is to burn the fuel as completely and rapidly as possible and under the highest temperature conditions so that no hydrocarbons, carbon or other particulars or carbon monoxide remain unburned. Although this method desirably yielded exhaust gases which are high in harmless carbon dioxide and relatively low in noxious carbon monoxide, it also resulted in high noxious nitrogen oxide levels in the exhaust gas. Thus, this prior practice was self-defeating insofar as reduction of carbon monoxide and nitrogen oxides are concerned since a reduction in one usually occurs at the expense of an increase in the other. It is evident, therefore, that air pollution control standards for maximum atmospheric emission of these pollutants are not easily met by burning fuel in the traditional manner.
Nitrogen oxides that appear in the exhaust gases result from oxidation of nitrogen in the air which is required for combustion and from nitrogen in the fuel itself. Most fuels contain some nitrogen compounds and heavy hydrocarbon fuels that are used for commercial purposes are particularly rich in nitrogen compounds which are at least partially converted to nitrogen oxides under high temperature combustion conditions. According to conventional practice, all of the fuel is burned as rapidly and as intensely as possible to produce gases as hot as 3200.degree. F. which is desirable for minimizing unoxidized components in the exhaust gases but this increases nitrogen oxides as stated.
The present invention is based in part on the recognition that if combustion occurs at above 1900.degree. F. and below about 2600.degree. F. substantially all of the hydrocarbons, carbonaceous particulates and carbon monoxide will be oxidized but a minimum of nitrogen oxides are produced.